The present invention relates to sheets of fibrous material, especially comprising cellulosic fibers, which are absorbent for aqueous liquids, and which are protected on at least one side by a membrane of permeable plastic.
There is great demand for materials which are capable of absorbing quantities of liquid, while remaining substantially solid, and which, before use, are compact. Examples of uses for such materials include kitchen rolls, sanitary pads, nappies, plasters and wound dressings in general.
Various materials are known which can be used for the above applications. Such materials include foamed plastics, absorbent paper and, more recently, sheets of cross-linked cellulosic fibers.
However, materials which are generally considered suitable do not necessarily fulfil all of the requirements for the application for which they are intended. For example, sanitary pads may be too bulky or too solid, and surgical dressings do not absorb, or do not absorb at a sufficient rate, a sufficient quantity of exudate from the wounds.
An added complication is that, for applications involving contact with the human or animal body, especially a wound, it is highly desirable that there be no toxic compounds present in the dressing which may affect the body in any way. This is a particular disadvantage of many plastics.
Materials which find general acceptance for most of the above-described applications comprise an interlinked, or cross-linked, disperse layer of a fibrous substance, such as cellulose, which has been so treated as to be able to absorb liquids. For example, EP-A-252650 discloses a material made of stiffened curled cellulosic fibers, while EP-A-7134 discloses a fibrous cellulosic sheet, the fibers being bibulous cellulosic fibers and/or sodium carboxymethyl cellulose fibers. These are cross-linked by wetting and applying heat and pressure.
One advantage of the cross-linked cellulosic fibers is their non-toxicity, provided that the cross-linker is a suitable non-toxic compound, such as carboxymethyl cellulose. These materials also have the advantage of being able to absorb up to about one hundred times their own weight in water.
A disadvantage of the cross-linked cellulosic materials arises through the various methods of production available for them. So far, no method for their production has been described which provides a highly absorbent matrix, the maximum being about 100.times. the mass of the matrix. This is essentially because of the difficulties involved in evenly distributing the cross-linker precursor throughout the fibers before effecting cross-linking. Two basic methods are known for the production of such matrices, the first of which is a dry process, and the second is a wet slurry process.
In the dry process, a layer of suitable cellulosic fibers is generated, such as by the air-felt process, followed by dredging a suitable powdered cross-linker onto the sheet and then compressing the whole, optionally after agitation, together with heating. It is generally necessary to use great pressure in order to effect any kind of satisfactory permeation of the cross-linker through the sheet, and the result is a very densely compressed sheet with variable concentrations of cross-linker throughout. These sheets tend to be least absorbent.
The alternative, wet process involves making a slurry of the cellulosic fibers and the cross-linker. This slurry is dried out and formed into a sheet, and then compressed and heated as before. This results in a more even distribution of the cross-linker throughout the material, but still does not form an optimal material with a particularly even density of cross-linker throughout, and also suffers from the drawback of being time consuming. The main problem is clumping with materials prepared from slurries, even where relatively low quantities of cross-linker are used.
Still further problems arise. While, for some applications, it is suitable to apply the material directly to the area where it is required to absorb liquid, this is, as a rule, undesirable for the treatment of wounds, as the fibers may irritate, or penetrate the wound, or both. In addition, exposed fibrous matrices, especially those containing wound exudates, can provide an ideal environment for the reproduction of bacteria.
To overcome such problems, articles comprising such materials generally further comprise protective membranes. In such instances, it is common to have an impervious membrane on the side of the material not intended for absorption, while a porous membrane is provided on the absorptive side. Well known examples of this type of application include nappies and sticking plasters.
Whilst it is relatively easy to provide a porous membrane to allow the cross passage of liquids, problems arise in preventing backflow of the liquids. In sticking plasters, for example, the porous membrane characteristically comprises an impervious plastics film with many small perforations, but this can allow backflow of the exudate which generally results in the plaster becoming embedded in the wound and, when the plaster is removed, reopening the wound, readily leading to the production of scar tissue.